In the field of rubber industry, especially, automobile components are desired to have advanced function and performance. Rubber products for use for such automobile components include frictional power transmission belts and the frictional power transmission belts are widely used, for example, for power transmission for driving auxiliary machine such as automobile air compressors or alternators.
As a belt of the type, there is known a V-ribbed belt that has ribs provided in the belt length direction. As the V-ribbed belt, these days required is a V-ribbed belt that satisfies quietness (noise suppression in dry and in pouring water) or fuel efficiency, especially satisfying both these performances.
Regarding quietness (silentness), for example, Patent Document 1 discloses a power transmission belt in which an ethylene-α-olefin copolymer rubber is used as a base rubber for a compressive layer, carbon black is contained in an amount of at least 50 parts by weight (from 50 to 100 parts by weight) relative to 100 parts by weight of the base rubber, and the carbon black contains large-sized particles of carbon black having an iodine adsorption of at most 40 mg/g, in an amount of at least 30 parts by weight relative to 100 parts by weight of the rubber. The power transmission belt of Patent Document 1 is specialized to the quietness thereof in dry, but nothing is taken into consideration at all relative to the quietness thereof in pouring water, and therefore, there is a drawback of noise generation in pouring water.
Patent Document 2 discloses a frictional power transmission belt of which at least the frictional power transmission surface is formed of a rubber composition that contains a surfactant blended in an amount of from 1 to 25 parts by weight relative to 100 parts by weight of the ethylene-α-olefin elastomer therein. In the frictional power transmission belt of Patent Document 2, the surfactant blended can improve the affinity between the rubber (ethylene-α-olefin elastomer) to form the frictional power transmission surface and water, therefore reducing noise generation owing to friction of misalignment or the like and to thereby improve quietness. However, even if the affinity for water could be improved, a water film would be still constantly formed between the frictional power transmission surface and pulleys, and therefore the belt would still remain to be in a slippery condition and could not be said to be sufficient in point of the quietness and power transmission performance thereof in pouring water.
Patent Document 3 discloses a V-ribbed belt in which a compressive layer contains cotton short fibers and para-aramid short fibers which are formed to protrude from the side of ribs and in which the protruding para-aramid short fibers are made fibrillated. In the V-ribbed belt of Patent Document 3, the cotton short fibers embedded in the compressive layer absorb the water existing between the pulley surface and the compressive rubber surface while, at the same time, the fibrillated para-aramid short fibers sweep away the water existing between the pulley surface and the compressive rubber surface, and in such a way, both the short fibers remove water to thereby prevent minor slips in pouring water. However, cotton short fibers are naturally twisted by themselves and are therefore difficult to disperse in a rubber composition. In particular, when a large amount of such cotton short fibers are blended, there is concern that they would cause dispersion failure, and from such a portion as a starting portion, a crack would occur. In addition, there is concern that such dispersion failure of cotton short fibers would be a factor to generate fluctuation in internal heat generation in the rubber composition (in that the internal heat generation differs between the cotton short fibers-aggregated part and dispersed part), therefore causing another problem of reduction in the fuel efficiency in belt running. To solve these problems, heretofore, there has been employed a method of prolonging the kneading time. For example, as described in Patent Document 4, there is known a method of putting cotton short fibers into a dispersion that contains a surfactant in an amount of from 0.5 to 15% by weight in water, followed by stirring and thereafter dewatering and drying, and blending the resultant fibers into rubber and dispersing them therein. This document discloses that an anionic surfactant is preferred. However, these methods take a lot of time and labor, therefore providing a factor of notably reducing productivity.
Regarding fuel efficiency, for example, Patent Document 5 discloses a V-ribbed belt in which a rubber composition to form a compressive layer contains an ethylene-α-olefin elastomer as a main component and in which the compressive layer has a Tan δ (loss tangent) at 40° C. of less than 0.150 when dynamic viscoelasticity is measured under conditions of an initial strain of 0.1%, a frequency of 10 Hz and a strain of 0.5%. In the V-ribbed belt of Patent Document 5, the compressive layer is formed of the rubber composition in which the ethylene-α-olefin elastomer content ratio is at least 45% by mass and the carbon black content ratio is less than 35% by mass, and consequently, Tan δ at 40° C. can be made low to reduce the internal loss to thereby enhance the fuel efficiency. However, when the polymer fraction is increased, then the friction coefficient at the frictional power transmission surface of the compressive layer tends to increase, therefore causing a risk of stick-slip generation and noise generation (in dry, in pouring water).
As in the above, the frictional power transmission belts of Patent Documents 1 to 3 and 5 could satisfy any one characteristic of quietness and fuel efficiency but could not satisfy both the two performances.